XG is Yamaha's extension of GM or General MIDI Level 1, also abbreviated GM1 to distinguish it from GM2 or General MIDI Level 2. GM has only 128 melodic voices and 1 drum kit, but XG adds the possibility of having 16,384 banks of voices. Yamaha created a number of standard XG voices; I'm not sure of the exact number, because different versions of XG had different numbers of voices-- that is, some versions added voices that weren't in previous versions-- but in the specifications for current models you'll generally see a reference to 480 XG voices.
There are two ways you can think about XG voices: (1) XG voices are "legacy" voices which use older sound samples than the more modern "panel" voices, so they generally don't sound as good as panel voices. (2) XG voices are used to ensure greater compatibility of song files and style files, since files that use XG voices should play on just about any XG-compatible keyboard. Note that some XG voices are considered to be standard, whereas others are considered to be "optional," meaning those optional XG voices can be omitted from a keyboard and the keyboard can still be said to be XG-compatible-- assuming it meets the other criteria for XG compatibility. XGlite is a "lite" or somewhat-reduced version of XG, such that some of the standard XG voices are optional in XGlite, and there are other reduced criteria, such as not having to respond to all of the Control Change messages that are required for full XG compatibility.
So what are panel voices? Basically, panel voices are "model-specific" voices which are not part of the XG voices, so two different XG-compatible keyboards might have different panel voices than each other. In practice, similar models tend to have a lot of the same panel voices, but there's no guarantee that one XG-compatible keyboard will have the same panel voices as another XG-compatible keyboard. What this means in a practical sense is that if you have song files and style files which are specifically designed for one series of keyboards, there's a good chance that many of the voices used in those files will be panel voices, and that the files might not be 100% compatible with another series. If that's the case, why use panel voices at all? Simply put, panel voices generally sound much better than XG voices. As said previously, they are newer voices and use better voice samples than the old XG voices.